The present inventors have developed a 4.7 GB DVD-RAM and a 25 GB, one-sided single-layer (1× speed) Blu-ray Disc as high-capacity, phase-change optical discs that can be used for data files and image files. And from the viewpoint of increasing the recording capacity of the optical discs, the present inventors further have developed a 50 GB (1× speed) Blu-ray Disc having two information layers on one side for the first time. These discs already have been commercialized.
A phase-change recording method, by which these DVD-RAM and Blu-ray discs are recorded, utilizes the fact that a recording layer reversibly changes its state between amorphous and crystalline (or between crystalline and another crystalline with a different structure) by being irradiated with a laser beam. Recording is performed by irradiating an information recording medium with a laser beam to change at least either one of the refractive index and the extinction coefficient of a thin film. In the recorded portion, the amplitude of transmitted light or reflected light changes, and as a result, the amount of the transmitted light or the reflected light changes when reaching a detecting system. The detecting system detects this change to reproduce signals. Generally, it is regarded as an unrecorded state when a recording layer material is in a crystalline state. Signals are recorded by irradiating an information recording medium with a laser beam, melting the recording layer material, and then rapidly cooling the recording layer materials to change it into an amorphous state. In order to erase the signals, the information recording medium is irradiated with a laser beam power lower than that used for recording to change the recording layer into a crystalline state. Phase-change optical discs generally have a dielectric layer, a recording layer and a reflective layer formed on a substrate. Examples of the configuration of such a disc include a configuration in which a first dielectric layer, a recording layer, a second dielectric layer and a reflective layer are stacked sequentially on a substrate.
The following explains the role of each layer. The role of the dielectric layer includes protecting the recording layer from mechanical damage from the outside, emphasizing optical changes using interference effects due to multiple reflections, blocking the influence of outside air to prevent chemical changes, and reducing roughening of the substrate surface and thermal damage of the recording layer in the case where signals are recorded repeatedly. The dielectric layer sometimes is referred to as a protective layer. Moreover, the speed of the crystalline-amorphous state change of the recording layer depends mainly on the composition of the dielectric layer (also referred to as an interface layer), which is in contact with the recording layer. Thus, the dielectric layer has another important role in controlling a crystallization rate. The recording layer absorbs the laser beam as previously described and undergoes the crystalline-amorphous state change, and thereby information is recorded on the recording layer. The role of the reflective layer is to diffuse the heat generated from the recording layer that becomes hot through the laser beam absorption while recording and erasing information.
The present inventors have developed a single-sided dual-layer Blu-ray disc. Single-sided multilayer discs typified by a single-sided dual-layer disc have a plurality of information layers (a first information layer 2, a second information layer 4, a third information layer 6, . . . , an n-th information layer 8 (where n is an integer of 4 or more)) on a substrate 1 as shown in FIG. 2. Each of transparent optical separation layers 3, 5, and 7, which are formed using an ultraviolet curable resin or the like, is interposed between the information layers (between an information layer and another information layer) that are adjacent to each other. Further on top of it (on the n-th information layer 8 in the structure shown in FIG. 2), a cover layer 9 (an optically transparent layer) formed using, for example, the ultraviolet curable resin is provided. In order to record or reproduce information on or from each of the information layers, a laser beam 10 is incident on each of the information layers from the side of the cover layer 9.
Current requirements for information recording media are to have an increased capacity and being available for increased speeds (recording and reproducing information at higher speeds). Developments are proceeding also for Blu-ray disc with an aim of recording and reproducing at a speed within a range of 1× to 2× (9.8 m/s linear velocity) or a range of 1× to 4× (19.7 m/s linear velocity) with respect to the 1× (4.9 m/s linear velocity) recording that already has been commercialized. The present inventors are currently developing a disc to be available for a speed of 1× to 2×.
In phase-change recording, the crystallization rate is adjusted by changing the composition of the recording layer according to a specified linear velocity. When the linear velocity is high, the crystallization rate is increased to make erasure easy. This, however, tends to spoil the stability of recording marks (amorphous phase). When the linear velocity is low, recording becomes easy by decreasing the crystallization rate, but the amorphous phase becomes too stable and thereby a reliability problem arises in that erasure becomes difficult. In order to solve such a problem associated with high speed discs, the present inventors have found Ge—Bi—Te—M (M denotes at least one element selected from Al, Ga, In, and Mn) as a composition for the recording layer.
The higher the linear velocity becomes, the more laser power (recording sensitivity) is required for recording information. Under such a condition, in order to solve the above-mentioned problem and to improve repetitive rewriting performance, an oxide-based material layer containing Hf or a mixture of Zr and Hf, at least one element selected from a group consisting of La, Ce, Al, Ga, In, Mg and Y, and oxygen is disclosed as a dielectric layer (including an interface layer) that has been reported so far (refer to JP2005-56545A, for example).
In this way, the compositions of the materials for the recording layer and the dielectric layer (including the interface layer) are being studied in order to ensure recording and reproducing properties and reliability (repetitive recording) as the speeds become higher.
The following problem arose in increasing the speed of the Blu-ray disc (1× to 2×) that has been under development by the present inventors.
A high temperature humidity test (moisture resistance test), which is one of the lifetime tests, was conducted on a disc. For the recording layer thereof, a Ge—Bi—Te—M material, where M denotes at least one element selected from Al, Ga, In, and Mn, was used. For the layer (the dielectric layer (the interface layer)) that is in contact with the recording layer, a material containing a Zr oxide as its main component was used. A problem however, arose that the recording layer was separated from that layer (the dielectric layer (interface layer)). In order to deal with this problem, various film forming conditions (sputtering power, sputtering gas pressure and additive gas) were examined for the recording layer and the dielectric layer (interface layer), but the separation could not be suppressed.